Well, for a unicorn pony like myself, it's a simple matter of biology. The horn converts mental and physical energy and willpower into magical effects that I admittedly can't explain very well. Pegasi like Rainbow Dash and earth ponies have use magic as well, but for them it's more passive, giving them the ability to manipulate weather and fly [pause for breath] and great physical strength and affinity for plant life, respectively.

I think it's mostly because of that that I've had no difficulty with it at all.